DE20112599U1 - MDF board and manufacture - Google Patents

Description

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MDF board and manufacturing

The invention relates to a manufacturing process and an associated
Device for a board made of MDF - called "MDF board" and a board manufactured according to the process.

A typical, well-known production process for the manufacture of a board of the type mentioned above is carried out as follows. Cooked wood chips are first fed to a so-called refiner to produce the MDF board. In the refiner, the wood chips are processed into fibers by adding temperature and pressure using painting discs. The fibers are transported out of the refiner using steam and passed on via a line called the "blue line". The steam pressure is around 10 bar. The temperature is around 150 to 160 °. Glue is added to the "blue line". After the glue has been added, the "blue line" expands. The expansion causes turbulence. The glue mixes with the fibers. The glue content in relation to the fibers is around 22% by weight.

The "Blue Line" ends in the middle of a drying tube. The drying tube has a diameter of 2.60 m, for example. Air with a temperature of 160° C, maximum of 220 to 240 ⁰ C, is blown through the drying tube. In the drying tube, the humidity is reduced from 100% to 8 to 11%.

In the drying tube in particular, the glue is exposed to undesirable temperature treatment. At temperatures above 80°C, glue is adversely affected or activated. Activated glue can no longer be used in the subsequent processing step, in which the glued fibers are pressed into the board.

02-0S-O i

The above-mentioned state of the art reduces the active part of the glue. Of the original 22% by weight, only 14% by weight is still ready for use according to the described state of the art when the fiber-glue mixture leaves the drying tube.
5

A formaldehyde-urea-based glue is currently used for MDF boards and chipboards. If boards are made for flooring, melanin is added to the glue. This is intended to prevent swelling that can occur due to moisture.

The problem is that part of the glue is lost during the heat treatment for the actual processing step. The disadvantage is that much more glue has to be added to the fibers than is necessary to press the fibers in a press with the addition of heat and thus achieve the desired result, i.e. the MDF board. Currently, an MDF board contains around 60 kg of glue per m ³ .

The object of the invention is to create an MDF board with a lower glue content compared to the prior art.

The object of the invention is achieved by a method having the features of the main claim. This results in a plate having the features of the secondary claim. Advantageous embodiments emerge from the subclaims.

To achieve the object of the invention, the fibers are first dried and then glue is mixed with the dried fibers.

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This prevents the glue from being undesirably exposed to the relatively hot temperatures that occur during drying.

Another advantage is that only water, not chemicals, are dried in the dryer or drying tube. This has environmental benefits, as the drying air is not disadvantageously contaminated with fumes that, according to the state of the art, come from the glue.

The fibers that are dried are advantageously not contaminated with glue.

Glue "disturbs" the drying process. Compared to the state of the art, the dryer therefore saves considerable amounts of energy that would otherwise have to be used for drying. This results in significant cost advantages.

The gluing process according to the invention reduces the amount of glue required in the field of MDF boards. A reduction to 45 to 55 kg per m ³ of board is achieved. A typical value is 50 to 52 kg per m ³ of board.

An essential factor in achieving the appropriate gluing of fibers is the "correct" ratio of fibers to glue. According to the invention, in one embodiment of the method, the dried fibers are fed to a belt scale before gluing.

On the belt scale, the fibers are transported on one side by means of a circulating conveyor belt, and on the other side they are weighed. This provides information on how much glue needs to be added to the fibers in the next step.

It is not easy to apply sufficient glue to fibres, as fibres tend to bunch up like cotton wool. It is then difficult to distribute the glue evenly over the fibres. In one embodiment

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According to the invention, the gluing is carried out in a mixer in which the glue and fibres are mixed together.

In one embodiment of the invention, the mixer has means for cooling its housing. In a particularly simple embodiment, an at least partially double-walled housing is provided for this purpose, for example a double-walled pipe, which is part of the mixer housing. A cooled liquid is passed through the double-walled housing to cool the mixer or its walls. The cooling should create a layer of condensation on the walls inside. The cooling should be designed accordingly. The layer of condensation ensures that glued and glue-free fibers do not stick to the walls and clog the mixer.

After the fibers have dried, in one embodiment of the invention they are spread out over the surface, forming a kind of curtain from the fibers. Glue is then added, in particular sprayed into the curtain. Preferably an air-glue mixture is sprayed in to ensure that the glue is distributed as evenly as possible. By forming a curtain, the glue is distributed more evenly across the fibers than if the fibers were in the form of cotton wool.

In a further embodiment of the invention, the cotton wool formed from the fibers is introduced into the mixer in the form of a curtain. The curtain is then blown through nozzles with an air-glue mixture. The glue is thus fed to the curtain via the nozzles. The curtain is then guided through the mixer, preferably without contact.

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The glue is blown into the dried fibers together with air, particularly at a temperature of 40 to 70 ^° C, preferably at a temperature of 55 to 60°C. This ensures that the glue reaches a dry outer skin. It is therefore activated to a minimal extent. This improves the effect that the subsequent glue-fiber mixture does not stick to the transport system and equipment, for example inside the mixer.

In one embodiment of the invention, the glue is prepared in such a way that it hardens after a predetermined time. The glue can be adjusted to suit the requirements by temperature treatment. A hardener can also be added, which hardens after 60 seconds, for example. The preparation of the glue is carried out in particular in the mixer, or a hardener is added to the dried fibers together with the glue immediately before the mixer.

This has the advantage that when the fibers are later pressed together to form a board, the glue immediately solidifies quickly. This means that shorter pressing times can be achieved. This represents another significant economic advantage over the state of the art, where these short pressing times could not be achieved due to the required curing times of the glue.

Since the glue is exposed to much lower temperatures than before, it is possible to use more reactive glues than the state of the art. In addition, it is possible to reduce the amount of chemicals such as formaldehyde. This results in further environmental benefits.

In one embodiment of the invention, the glue is mixed with heated air and this air-glue mixture is added to the dried fibers. The warm air, which is fed together via a booth, for example,

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with the glue and the dried fibers is introduced into the mixer, activates the surfaces of the glue droplets produced slightly. This prevents fibers from sticking to subsequent equipment, such as mixer walls. Otherwise, the mixer would have to be cleaned in a very short time. Production would then be stopped, which would be disadvantageous. Undesirable cleaning costs would also be incurred accordingly. These considerable economic disadvantages can be compared with the disadvantage that glue is activated a little. However, the proportion of activated glue is low compared to the state of the art.

With a few tests, the specialist can determine how far the glue needs to be activated on its surface in order to achieve an optimal economic

After the glue has been added to the dried fibers, the free surface of the glue is further activated somewhat by a device suitable for this purpose in one embodiment of the invention in order to facilitate subsequent processing steps. After the glue has been added to the dried fibers, in particular after leaving the mixer, the fibers covered with glue therefore preferably end up in a riser pipe which is in particular 10 to 30 m, preferably around 20 m long. The diameter of the riser pipe is preferably 1 to 4 meters.

The riser pipe is preferably also cooled and is then, for example, double-walled in order to pass a cooling liquid between the two walls of a double wall. The aim is again to form a layer of condensation on the inner walls of the riser pipe so that the glued fibers do not stick to the walls.

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It has been found that the fibers should be blown through the riser at a speed of at least 25 m/sec, preferably at least 35 meters per second. If the speed is lower, fibers will stick to the riser despite the measures mentioned above. This would contaminate the riser. When lower speeds were used, the riser had to be cleaned after just 8 hours. By setting a suitable speed, the cycles could be extended to 7 to 8 days. The riser only needs to be cleaned once a week.

The maximum speed at which the glue-coated fibers are blown through the riser pipe depends on the performance of the downstream components. It must be taken into account that the downstream components must be able to process the incoming quantity of fibers. In practice, an upper limit of 40 meters per second can currently be achieved without any problems. Above 50 meters per second, the downstream components used to date were overloaded.

By providing a riser pipe, the glue on the surface is further activated so that subsequent processing steps can be carried out appropriately.

After the addition of glue to the dried fibers, especially after the partial activation of the glue in the riser pipe, the fibers that are covered with glue enter a cyclone. Here, the glue has now been sufficiently activated on the surface due to the aforementioned measures, so that it no longer adheres to the cyclone. In the cyclone, the fibers are separated and fed to the next processing step with a belt. In one embodiment, the belt guides the fibers into a viewing device. In the

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The fibers are examined for coarse components using a viewing device. The coarse components are sorted out. Coarse components include lumps of glue, for example.

From the viewing device, the fibers are transported to the press by means of a belt, where they are pressed into the board. The press preferably consists of rotating press belts pressed against each other, which are heated to the appropriate temperature. This allows continuous pressing. The temperature must be adjusted by a specialist to suit the glue used.

The nozzles through which the glue is added to the fibers in one embodiment of the invention are preferably conical. The glue then emerges in droplets through the cone tip, thus promoting even distribution.

In order to avoid cleaning work and the associated downtime in production, it is advantageous if the glue emerging from the nozzles, for example, does not come into contact with subsequent tools, such as those in the mixer. The glue is therefore preferably sprayed directly in the direction of the fibers, for example. Furthermore, care must be taken to ensure that there is sufficient distance between nozzles and subsequent tools in a mixer, for example. In practice, it has been found that the distance between tools in the mixer and the nozzles should be at least 1 meter, preferably at least 2 meters, if the glue is injected horizontally. The fibers are then introduced vertically at the beginning of the mixer and transported further horizontally within it. The specific distance values mentioned naturally only refer to a specific individual case. They are not generally applicable, since the speed at which the glue emerges from the nozzles is ultimately also important.

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The tools used in a mixer are in particular stirring devices, which mix the fibres with the glue.

To achieve good results, the fibers are placed in front of the nozzles in the form of a curtain. In addition to the advantages already mentioned, this prevents glue from splashing into the mixer and contaminating the tools. Otherwise, the fibers would stick to the tools and the mixer would become clogged in no time.

The tools in the mixer are attached to a centrally installed axis and consist of star-shaped protruding rods that transition into a flat area, similar to a rudder blade. A star is formed from four tools in total, for example. Two tools each form an angle of 90°. The rudder blades are angled in comparison to the air flow that flows through the mixer. This causes the air to swirl and thus ensures that the fibers are well mixed with the glue. Several "stars" formed by tools are attached to the axis at regular intervals. The fibers are then transported through the mixer parallel to the axis.

A curtain is preferably produced from the fibers as follows.

A means of transport, such as a conveyor belt or a belt scale, is equipped with rollers at the end. The fibers are guided through the rollers. The rollers are pressed against each other.

This ensures that the rollers create a kind of curtain for the fibres. The curtain shape is thus determined by the rollers

generated.
30

A conveyor belt is preferably used, as this ensures a uniform supply of fibers to the rollers. According to

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According to the state of the art, screws are regularly used to transport fibers in the production of MDF boards. However, fibers leave screws relatively unevenly. The result is a correspondingly uneven curtain. A curtain that is evenly thick and wide is advantageous in order to achieve an even distribution of glue. This also ensures that the curtain reliably separates injected glue from subsequent tools.

In particular, the pressed rollers used to create the curtain prevent the fibers from being passed on in a wadding or lump-like manner. This would hinder the desired uniform gluing.

In order to be able to process a sufficiently large amount of fibers into a curtain and to achieve a particularly uniform curtain, in one embodiment more than two rollers are pressed against each other through which the fibers are passed to produce a curtain. They are preferably arranged offset one above the other so that an acute angle of the rollers with a transport means, such as a conveyor belt or the

This allows enough material to be added to the means of transport, for example to the belt scale, in order to be able to process a sufficiently large quantity evenly.
25

In practice, it has been shown that a total of four rollers are particularly advantageous for producing a curtain from the fibers, which is then mechanically glued.

The opening through which the curtain consisting of fibres is introduced into or in front of the mixer in one embodiment preferably corresponds to the maximum width of the mixer housing, for example

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the diameter of the pipe mentioned, which also forms the walls of the mixer. This ensures that the entire width of the mixer is covered by the curtain. Otherwise, glue could spray past the curtain at the remaining openings into the interior of the mixer, causing the aforementioned contamination problems.

If the entire width of the mixer were not covered, not only would glue spray into the mixer, but more edge fibers would be carried along and clump together. This would impair the quality of the material. Corresponding production problems and the material would have to be reprocessed, which would be disadvantageous and costly.

In practice, the side walls of the mixer are preferably cooled to 7 to 15 ^° C, especially to 10 to 12°C. This ensures that a layer of condensation water settles on the walls. The layer of condensation water prevents sticking.

The temperatures mentioned are also suitable for the formation of a layer of condensation on the inner walls of the riser pipe.

In a further embodiment of the invention, wood chips or wood shavings are first separated into the solid wood component cellulose and broken down into the liquid components lignin and liquid hemicellulose. Lignin and hemicellulose are separated from the solid components and used as glue, i.e. mixed with the dried wood fibers according to the invention. The solid wood components are further processed into fibers. The liquid components can be separated from the solid components in a so-called agitator. The aforementioned components that are obtained are typically

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at: 20 to 35 wt.% hemicellulose, 45 to 50 wt.% cellulose and 20 to 35 wt.% lignin.

In one embodiment, the wood chips are first fed into a screw conveyor. From the screw conveyor, the wood chips are compressed and fed into a cooking container, where they are cooked at high pressure. The cooking container is designed to withstand high pressures. The pressure in the cooking container is at least 16 bar. According to the state of the art, wood chips are usually cooked at pressures of just 13 to 15 bar. The thermal steam treatment separates the solid wood components (cellulose) from the lignin and hemicellulose. The cellulose is in solid form. The other two components, lignin and hemicellulose, are liquid and can generally be used as glue. The adhesive strength is mainly provided by the hemicellulose.

It is known from the publication WO 98/37147 that the lignin and hemicellulose contained in wood can be separated from the solid components and then used as glue in the manufacture of MDF boards. The disadvantage of this process was that it produced strong odors that would have polluted the environment of a production facility to an unacceptable extent. The development of odors could not be eliminated by economically viable measures. The problem of odor development is eliminated according to the invention by the fact that the liquid components are initially located in the pressure-tight cooking container from which no odors can escape. After the liquid components have been separated, they cool down and are further processed at relatively low temperatures, in particular by spraying them onto the fibers via nozzles. The liquid components are therefore significantly cooled before they leave the odor-tight encapsulated system. In this relatively cool state, the

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Odor development is very low. The use of lignin and hemicellulose as glue is made possible by the fact that these components of wood only leave an odor-tight encapsulated system at low temperatures, particularly at temperatures well below 100 ⁰ C, and are applied to the fibers in this cool state. In this way, it is possible to reduce the environmental impact sufficiently in an economical way.

In one embodiment of the invention, the liquid portions of hemicellulose and lignin obtained in the manner described above are mixed with conventional glue. The proportion of hemicellulose and lignin in the glue mixture is preferably not more than 20% by weight. The mixture also contains, in particular, a formaldehyde-urea-based glue.

If a glue mixture is used that contains more than 20% by weight of hemicellulose and lignin, the pressing time during which the glued fibers are pressed into the board will be too long. It is therefore more economical to mix hemicellulose and lignin with another glue. In this way, conventional glue can be saved and the process does not become too long due to long pressing times. The upper limit for the proportions of hemicellulose and lignin that makes economic sense naturally depends on the reactivity of the glue with which these components are mixed. The value of 20% by weight mentioned is therefore only a guideline.

Since air is provided for transporting the fibers with the glue through the mixer, the nozzles for feeding glue in one embodiment of the invention are spaced apart from the mixer housing. The nozzles are then located in front of an opening in the mixer housing. This leaves an annular gap between the nozzles and the opening, through which air can be entrained and thus suitably supplied.

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Furthermore, in this embodiment, the air introduced via the annular gap can be preheated to provide a desired temperature in the mixer.

In one embodiment, tools inside the mixer are mounted on an axis. The nozzles for feeding glue are then arranged in a ring around the axis in order to evenly cover the fibers with glue. The fibers or the curtain made of fibers are then preferably fed perpendicular to the axis between the nozzles and tools. Depending on the diameter of the mixer, nozzles are arranged in one or more rows in a ring. If the diameter is large enough, the entire opening of the mixer is sprayed with glue by arranging a second row of nozzles in a ring around the axis.

In one embodiment of the invention, glass fibers or plastic fibers are added to the fibers consisting of solid wood components. The addition takes place in particular in or immediately before the mixer. This makes it particularly easy to produce plate-like molded parts that are intended, for example, as interior paneling in a car. Such molded plates can be used in the automotive industry, for example, as a parcel shelf. It is then sufficient to simply pre-press the layer system. A final pressing step does not have to be carried out.

The automotive industry does not require as many moulded parts as the fibres that can usually be produced economically on a large industrial scale. It is therefore more economical to produce moulded parts, which are used in particular in the automotive industry, together with MDF boards intended for the production of panels in order to be able to use the fibre quantities on a large industrial scale. The MDF boards intended for the production of panels have a

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They have a top and a bottom that run parallel to each other and are flat. These panels are a few millimeters thick. They usually do not contain any plastic or glass fibers, as no special shapes have to be created that deviate from a flat surface. 5

Sharp edges are problematic when manufacturing molded parts. They tend to tear. These problems can be avoided by reinforcing them with glass fiber or plastic fibers.

Moulded parts of the aforementioned type are also used in the furniture industry. Such moulded parts are required, for example, for doors that have a special shape for design reasons.

The invention is further illustrated by the following figures.

Figure 1 shows a section through a belt scale 1 and a downstream mixer 2. As indicated by the arrow 3, dried fibers made from wood chips are fed to the belt scale 1 through an opening in a housing 4. A slope 5 directs the incoming fibers onto the belt of the belt scale. The belt scale records the amount of material that is transported in the direction of the three rollers 6. The three rollers are arranged one above the other and offset so that they form an acute angle α with the belt scale 1. The fibers on the belt scale enter this acute angle. They pass the rotating rollers 6. The fibers form a curtain, which is transported vertically downwards along the arrow 7 due to gravity. The curtain thus enters the mixer 2 between nozzles 8 and tools 9.

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The mixer consists of a tubular housing. The housing is formed by a double wall 10 and 11. An axis 12 is arranged centrally inside the housing, on which the tools 9 are attached. A tool 9 forms a right angle with the axis 12. Four oar-blade-like tools 9 are grouped together in a star shape. Several of these grouped tools are attached to the axis 12 at equal intervals. The front area, into which the curtain made of fibers is introduced, is free of tools. This ensures that there is a sufficiently large distance between the tools 9 and the nozzles 8.

This distance is provided so that glue escaping from the nozzles 8 does not directly hit the tools during operation.

The diameter of the housing of the mixer corresponds to the width of the opening through which the curtain made of fibers is introduced into the mixer. The width of the curtain is adapted to the width of the opening. The nozzles 8 are arranged in a semicircle around the axis 12 in an upper area. This ensures that, on the one hand, the curtain is evenly coated with glue and, on the other hand, the glue emerging from the nozzles 8 does not directly hit parts of the mixer. A gap is arranged between the nozzles 8 and the housing 10, 11 so that a kind of annular gap is formed. Air is sucked in through this annular gap. Means for heating the air that is sucked in are not shown. This creates a glue-air mixture. The curtain coated with glue (in other words, a mat made of fibers) is transported through the mixer 2 by the air flow parallel to the axis 12. The axis rotates during transport and so do the tools 9. The glue is further mixed with the fibers. A cooled liquid is introduced between the two walls 10 and 11 of the double wall in order to create a layer of condensate on the inner walls of the mixer.

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Figure 2 shows a top view of the mixer parallel to the axis 12. For reasons of clarity, only two tools 9 are shown. Figure 2 particularly illustrates a single-row, semicircular arrangement of the nozzles in the upper area.

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Claims (23)

1. Device for producing a board made of MDF with a drying device in which the fibers are dried, and with a gluing device in which the fibers are provided with glue, and with means for pressing the glued fibers into a board, characterized in that transport means ( 1 , 7 ) are provided with which the fibers are transported from the drying device to the gluing device ( 2 , 8 ). 2. Device according to claim 1 with a device in which wood chips are processed into fibers, in particular by supplying temperature and pressure with the aid of painting disks. 3. Device according to claim 1 or 2, wherein the drying device comprises a tube and means by which a gaseous medium is heated and blown through the tube. 4. Device according to one of the preceding claims, wherein the transport means comprises a belt scale ( 1 ). 5. Device according to one of the preceding claims, in which a mixer ( 2 ) is provided in which glue and fibers are mixed with one another, in particular mechanically by means of stirring tools ( 9 ), wherein the stirring tools are preferably arranged in the manner of rudder blades and propellers in order to be able to cause a swirling of air in the mixer. 6. Device according to one of the preceding claims, in which a mixer ( 2 ) is provided together with means for cooling its housing ( 10 , 11 ). 7. Device according to one of the preceding claims, in which a mixer ( 2 ) is provided which at least partially comprises a double-walled housing ( 10 , 11 ), in particular a double-walled tube. 8. Device according to one of the preceding claims, in which cooling means are provided for cooling a liquid, and means for cooling the housing of a mixer and/or a riser pipe with the cooled liquid. 9. Device according to one of the preceding claims, with means for producing a layer of condensate on the inner walls of a mixer and/or a riser pipe. 10. Device according to one of the preceding claims, with means ( 6 ) for feeding the fibres in the form of a curtain or a mat to the gluing device. 11. Device according to one of the preceding claims, with means ( 6 ) for feeding the fibers in the form of a curtain or a mat to the gluing device, these means comprising rollers ( 6 ) and a conveyor belt or a belt scale ( 1 ) being provided for feeding fibers to the rollers. 12. Device according to one of the preceding claims, with means ( 6 ) for feeding the fibres in the form of a curtain or a mat to the gluing device, said means comprising rollers ( 6 ) which are arranged one above the other and offset, the rollers in particular being arranged such that they form an acute angle (α) with a conveyor belt or a belt scale ( 1 ). 13. Device according to one of the preceding claims, with nozzles ( 8 ) through which glue is applied to the fibers, which are in particular conical. 14. Apparatus according to any preceding claim, comprising means for applying glue to the fibres together with heated air. 15. Device according to one of the preceding claims, with means for applying glue together with a hardener to the fibers. 16. Device according to one of the preceding claims, with a substantially vertically extending riser pipe which is connected to the gluing device and through which the glued fibers are blown against gravity, wherein means for cooling the walls of the riser pipe are preferably provided. 17. Device according to one of the preceding claims, with a cyclone in which glue-coated fibres are separated and/or a viewing device by means of which the glue-coated fibres can be optically inspected. 18. Device according to one of the preceding claims, with a press which comprises rotating press belts pressed against each other. 19. Device according to one of the preceding claims, with means for bringing fibres in the form of a curtain or a mat in front of the nozzles from which glue emerges. 20. Device according to one of the preceding claims, with a mixer ( 2 ) and an opening through which a curtain consisting of fibers is introduced into or in front of the mixer, wherein the opening corresponds to the maximum width of the mixer housing and the means for producing the curtain are preferably dimensioned such that the width of the curtain essentially corresponds to the width of the opening. 21. Device according to one of the preceding claims, in which a riser pipe made of metal and/or a mixer made of metal is provided. 22. Apparatus according to any preceding claim, wherein means are provided for separating wood into solid and liquid components and means for applying liquid components to the fibres. 23. Board, in particular producible with a device according to one of the preceding claims, characterized in that the proportion of glue in the board is 45 to 55 kg per m ³ , in particular 50 to 52 kg per m ³ .